EWING – A deep, concussive thud echoed through a township park this morning as authorities demonstrated the danger posed by crude explosive devices of the type that were illegally detonated twice in the township last month.

Two bottle bombs exploded in the West Trenton section of town in late May, and police are still trying to identify the culprits. In the meantime they are urging residents to observe extreme caution around suspicious bottles and objects that could be explosive.

“They’ll get chemical burns from chemicals inside, and it’s possible they could lose a finger,” New Jersey State Police Lt. Dwayne Jones said.

Jones and members of the state police’s Arson/Bomb unit were called out by township police to conduct the demonstration and warn about the dangers of fireworks.

First, a watermelon was annihilated by a small firework known as a “quarter stick,” sending pieces of fruit flying towards observers 150 feet away. One piece nearly clipped the top of Mayor Bert Steinmann’s head.

The demonstration device was a simple mixture of dry ice and water in a plastic bottle that closely mimicked the effect of the devices exploded in Ewing. The bottle began to bulge after the items were mixed, creating a white foamy substance inside and making the bottled roll around on the stump it was positioned on for more than a minute.

When it finally went off, the exploding bottled emiited a blast as loud as the firework and spewed out the chemicals inside. The bottle itself was blown apart into a single piece of melted, twisted plastic.

Police have some leads on who put together the two bombs they are improvised explosive devices (IEDs). They said residents should be on the lookout for suspicious activity in their neighborhoods, possibly involving juveniles.

“And we really encourage parents to keep an eye on their household chemicals,” said Ewing Sgt. John Stemler, who is handling the case.

Both previous IED detonations took place away from people, leading authorities to suspect the would-be bombers do not want to hurt anyone. But accidents are still possible with these volatile compounds, they say.

“The problem with these, too, if that you never know when they’re going to go off,” Stemler said.